Radio-frequency power measurement



NOV. 26, 1945. s, RAMO RADIO FREQUENCY POWER MEASUREMENT Filed Jan. l, 1943 RESISTANCE NEASURING DEVICE MICROWAVE POWER BRI DGE Patented Nov. 26, vv1946 RADIO-FREQUENCY POWER MEASUREMENT vvSimon Ramo, iskayuna,

eraVEIectric Company, a

York l N. Y.,`assignor to Gencorporation of New v Application January 1, 1943, Serial No. 470,976

z'oianns.i (o1. r11-ssi My invention relates to radio frequency apparatus and concerns particularly arrangements for measurement and comparison of power.

It is an object of my invention to provide arrangements for determination of the magnitude of alternatingcurrent power, especially radio frequency power in the microwave region although thejpower level to be measured is extremely low.y

Another object of my invention is to provide arrangements for determination of the noise level in radio apparatus.

Still another object of my invention is to provide arrangements for comparing relatively high level and low level power.

It is also an object of my invention to provide arrangements Afor measuring low level power with substantially the same accuracy as power may be measured at higher levels.

Other and further objects and advantages will become apparent as the description proceeds.

My invention finds its principal use in the eld of microwave measurements, that is, measurements of radio frequency energy where the wave length is of the order of centimeters, since in this region conventional wattmeters cannot be employed and previously known power measuring devices adapted to measurements in this range of frequencies do not give accurate results in case of low levels of power which must frequently be measured.

In carryingout my invention in its preferred form in power input of vmicrowave alternating current energy, I provide a radio frequency signal source, a pair of adjustable attenuators towhich the radio frequency signal source is coupled and Aoutput connections from the attenuators,. one of which leads to a device the power characteristics of which are to be measured at low level and the other of which leads to a power measuring device which operates on microwaves but which is `designed primarily for relatively high power measurements in order to obtain the desired accuracy. The attenuators are so adjusted that the power input to the device to be measured or tested is much more greatly attenuated thanfthe power input to the high level power measuring device. The attenuators are made as nearly as possible identical in construction and arrangementV so that the ratio of attenuation may be read from the settings of the attenuators althoughthe actual attenuation may be unknown. f

A better understanding of the invention will be afforded by the following description considered in connection with the accompanying drawconnection with the measurement of the ing, and those vfeatures of -the invention which are believed to be novel and patentable will be pointedl out inthe claims appended hereto.

The single figure of the drawing is a schematic diagram of one embodiment of my invention.

For the sake of illustration the invention will be described as used for the determination of the power input at a low level to a device such as a radio receiver II, for example, or an element of a radio receiver represented by one of the rectangles in the larger rectangle II. The manner of measuring the noise level in such a receiver or receiver component will al-so be explained.

For the purpose of making determinations a source of energy input of the desired frequency .is provided. This is represented by the box I2 captioned. A comparator attenuator I3 is prowded which is in the form of a pair of identical attenuators I4 and I5 arranged to have power from the RF signal source I2 supplied thereto through suitable connections such as a coaxial line I6. The attenuators I4 and I5 are -so constructed that the ratio of attenuation may be very accuratelyA known, although the absolute value of attenuation need not f be determined. Likewise it is unnecessary to be able to determine the value -of the power output of the RF signal source.

One of the attenuators; viz., the attenuator I5, has output connections in the form of a coaxial line I1, for example, to the device II, the power input to which is to be determined, or the noise level of which'is to be determined.

A suitable power'measuring device I8 is provided which need not be capable of functioning or measuring accurately, high level of power, in compari-son with that supplied to the device II.,y For thesake of convenience, therefore, I shall refer to the device I8 in the description and claims, as a high level power measuring device. It will be understood that by this expression I mean a power measuring device which mayv operate at a high level in comparison with the level of the power or the noise which is to be measured or determined. The output of the attenuator I4 is supplied tov the high level power measuring device I8 through suitable connection-s such as a coaxial line I9. The attenuators I4 and I5 are shown as being of the wave guide type in the form of hollow pipes which may be circular or rectangular, but for the sake of illustration, will be assumed to be circular. The

attenuators I4 and I5v are referred to as being of the wave guide type because no central conductor is provided but the dimensionssare such that they except at a relativelyv do not actually serve as wave guides but as attenuator-s instead, the transverse dimensions being so chosen that the cutoff wave length is less than the wave length of the energy supplied by the RF signal source I2. Under such conditions of operation the degree of attenuation is very high, the energy being attenuated to as little as onemillionth of the input energy ifdesired.

Since the attenuators I4 and I5 are identical, it will be suilicient to describe one of them in detail. The attenuator. I5, for example, comprises a tube or pipe 20 of considerable length in comparison with its radius and having a radius such that the cutoif frequency of the pipe 20 considered as a wave guide is less than the wave length of the source I2. For adjustment of the attenuation of the attenuator I3 a telescoping internal unit 2I is provided which is adapted to slide axially within the tube 20 and has a iiat annular end member 22 serving as an electrical termination for the pipe 20. 'I'he coaxial line I'I is extended into the member 2| so as to form a cylinder 23 and an inner conductor 24mounted within the slidable member 2I so as to move with it when the member 2| is slid to the right or the left for adjustment of attenuation. An attenuation scale 25 is provided-and there is a pointer 26 cooperating with the scale 25. The pointer 26 is mechanically connected to the member 2I so that it moves back and forth therewith and the position of the pointer 26 with respect to the scale 25 serves as an indication of the position of the electrical end point 22 of the attentuator pipe 20. The end of the internal conductor 24 may be extended to form a probe if desired, but this is not necessary since the source I2 ordinarily has suilcient power so that the power pickup of the concentric line elements 23 and 24 will be adequate.

For transferring power from the coaxial line I6 to the attenuators I4 and I5, a pair of probes 2'I and 28 may be providedwhich are-electrically connected to the internal conductor 29 of the coaxial line I6. The dimensions of the probes 2'I and 28 are not critical, but if it is desired `to make use of a signal source I2 of the minimum necessary power output, the probes 21 and 28 may be made one-fourth of a wave length. Furthermore, if it is desired to conserve power output, a conventional matching stub 30 may be provided having a movable end member 3I making electrical contact both with the continuation ofthe coaxial line internal conductor 29 and the external conductor 33.

I'he attenuator I4, as explained, is as far as possible identical with the attenuator I and has corresponding lements including a pointer' 34 cooperating with a scale 35.

The highflevel power measuring arrangement I8 may take the form of any suitable device for measurement of power in the microwave region such as a calibrated crystal, for example, con-A nected to a deecting instrument or may take the form of a bridge device of the type known as a microwave power bridge. Such microwave power bridges do not constitute my invention.

and carefully made end constructions 38 and 39, such that for practical purposes all of the power input received from the coaxial line I9 is transmitted to a resistor 40 at the end 39 of the element 36. The resistor 40 may take the form of a Littel fuse, for example. A suitable device, represented by the box 4I, is provided for both supplying a direct current to the resistor 40 and for measuring the resistance of the resistor 40. The direct current circuit comprises the device 4I, the conductor a, the resistorv40, the internal conductors b and c, the stub cylinder d, the cylinder e, and the return conductor f. Inasmuch as the current ilow through the resistor produces variations in its temperature and resistance, a measurement of the resistance of a resistor serves as a measurement of the DCwer input thereto. The high frequency power input to the resistor 40 may therefore be determined by comparing the resistance variations produced by the alternating current input power with the resistance variations produced by direct current input. Ordinarily a null arrangement, such as a Wheatstone bridge or the arrangement described in Patent No. 1,501,663, Hoxie, is employed. sired, the microwave power bridge may be adjusted for a predetermined power measurement, deviations from which will be indicated by deviation from a null indication in the device 4I. The fuse 40 and its holder are justable in tube 39.

When it is desired to make a measurement of the power input to the device II or of the power required to produce a predetermined effect in the device II, the apparatus is connected as shown and the attenuator I4 is so' adjusted that balance is obtained in the device 4I or a satisfactory measurement of power is obtained in the microwave power bridge I8. The power input to the device I8 is then known. The power input to the device II is determined by comparing the attenuations of the attenuators I4 and I5. Such scales 25 and 35. If these scales are uniformly graduated, the difn to ten times the logarithm of the ratio between the input power tion may be obtained from the following formula:

a is attenuation per unit length :wave length in meters `c=cutol wave length ic=1.640a

a=pipe radius in meters y posed between the units The methods and the apparatus hereinbefore described may be used for making various types of measurements involving determination of power. For example, if it is desired to determine the noise level of a radio receiver II having a power output meter 42 or connections to which a suitable measuring device such as a cathode ray oscilloscope may be connected for measuring power output, the noise level may be determined in the following manner. With no input signal, the reading of the device 42 is taken. An input signal is then applied, and the attenuator I is adjusted to give -double the former reading on the power output meter 42, the attenuator I4 being simultaneously adjusted to obtain a satisfactory reading on the microwave power bridge I8. The power input through the coaxial line I1 is obviously equal to the noise level of the device Il. This power input is calculated from the absolute value of power input measured by the microwave power bridge I8 and the ratio of attenuations measured in decibels and determined by the difference between the readings of the scales 25 and 35.

In case the device in which noise level is to be determined consists of a converter, or a converter and radio frequency amplier 43, the unit 43 will be combined with certain other units in a conventional manner so* that a power output reading may be obtained in the device 42. In the case assume'd a local oscillator 44 is provided, and other elements such as an intermediate frequency pre-amplifier 45, an adjustable gain amplifier 46 and a main intermediatefrequency amplifier 41 may be interposed between the converter 43 and the power output meter 42. In this case a three-decibel loss switch 48 may be inter- 46 and 41 or may be incorporated in the unit 41. Then the device 42 may be a null device which gives a null or zero reading for a predetermined power output from the main IF amplifier 41.

With the loss switch 48 disconnected and no input signal source applied to the line I1, the adjustable gain amplifier 46 is adjusted to obtain a null reading in the meter 42. Then the signal source is applied through the coaxial line I1, the loss switch 48 is thrown to the position in which it introduces a loss of three decibels, that is, a power reduction of one-half and the attenuator I5 is adjusted so that the meter 42 again reads zero. The attenuator I4 is simultaneously adjusted to obtain a satisfactory reading in the microwave power bridge I8. The power input through the coaxial line I1 determined from the 55 6 reading of the device I8 and the attenuation ratios of the attenuators I4 and I5 then equals the value of the noise level of the converter 43.

It will be understood that for noise level measurement, particularly, the adjustment of the attenuators I4 and I5 is such that attenuation of the unit I5 is much greater than that of the unit I4, thus permitting the measurement of very low noise levels although the device I8 may read satisfactorily only when very much higher levels of power are applied to it.

I have herein shown and particularly described certain embodiments of my invention and certain methods of operation embraced therein for the purpose of explaining its practice and showing -its application, but it will be obvious to thosel skilled in the art that many modifications and variations are possible, and I aim therefore to cover all such modifications and variations as fall within the scope of my invention which are defined in the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. Apparatus for measuring low level power which comprises a pair of attenuators of like construction such that the ratio of attenuation is determined by the differences in adjustment although the absolute attenuations in each may be unknown, means for simultaneously supplying power to both of said attenuators, means for conveying power output from one attenuator to a device in which low level power is to be measured, a high level power measuring device, and means for conveying power output from the other attenuator to said high level power measuring device.

2. Microwave low level power measuring apparatus comprising a pair of wave guide type attenuation pipes with means for supplying microwave energy thereto, said pipes having such transverse dimensions that the cutoff wave length is less than the wave length at which the power measurement is to be made, said attenuators having movable elements for varying the effective electrical lengths thereof to vary the attenuations and for determining the ratio of attenuation, a relatively high level power measuring device, means for conveying power output from one of said attenuators to a device in which low level power is to be measured, and means for conveying power output from the other of said attenuators to a relatively highv level power measuring device.

SIMON RAMO. 

